The immune systems of human and non-human primates have diverged over time, such that some species differ considerably in their susceptibility, symptoms, and survival of particular infectious diseases. Variation in primate immunity is such that major human pathogens - such as immunodeficiency viruses, herpesviruses and malaria-inducing species of - elicit striking differences in immune response between closely related species and within primate populations. These differences in immunity are the outcome of complex evolutionary processes that include interactions between the host, its pathogens and symbiont/commensal organisms. The success of some pathogens in establishing persistent infections in humans and other primates has been determined not just by the molecular evolution of the pathogen and its interactions with the host, but also by the evolution of primate behavior and ecology, microflora, immune factors and the evolution of other biological systems.